1. Field of the Invention
This invention is related in general to the field of orthopedic devices and methods for joining parts of fractured bones and/or soft tissue to bone cortex. In particular, the invention provides a method of performing these tasks by using a novel double-threaded screw and compression nut assembly.
2. Description of the Related Art
When bones are either fractured by accident or severed by surgical procedure, the healing process requires that they be rejoined and kept together for lengthy periods of time in order to permit the recalcification and bonding of the severed parts. Accordingly, adjoining parts of a severed or fractured bone are typically clamped together or attached to one another by means of a pin or a screw driven through the rejoined parts. Movement of the pertinent part of the body is then minimized by a cast, brace or splint in order to promote healing and avoid mechanical stresses that may cause the bone parts to come apart during normal or necessary bodily activity. The surgical procedure of attaching two or more parts of a bone with a pin-like device or screw requires an incision in the tissue surrounding the bone and the drilling of a hole through the bone parts to be joined, often with little space to operate without inflicting further injury to the patient. Therefore, it is very important that the arthrodesis device used be capable of quick installation with a simple procedure. U.S. Pat. Nos. 4,796,612 (1988) and No. 4,903,692 (1990) to Reese describe a bone clamp and a corresponding installation tool that enable a surgeon to easily drive a pin through an aperture in two or more parts of a bone and lock it in place by means of a self-locking button placed around the shaft of the pin. The tip of the pin has a hook that engages the distal cortical surface of the bone after protruding through the aperture in the bone. This bone clamp and installation tool provide an efficient method for clamping fractured bones together, but they have been found to be unsatisfactory when relatively great pressure is required to push the self-locking button toward the proximal bone cortex. In addition, the ability to make fine adjustments of the clamping force is limited by the step-like operation of the ratchet-type self-locking mechanism provided.
Accordingly, an improved device based on the same concept was developed and is described in my application Ser. No. 977,074, currently pending. This invention includes continuous threads over which the button may be either pushed forward or screwed, thus allowing for finer clamping pressure adjustments. A remaining problem arises from the fact that, in order to achieve these fine pressure adjustments, the button has to be rotated with respect to the proximal cortex over which it applies pressure. Thus, some unwanted scraping and damage to the adjacent tissue occurs.
The present invention is directed at solving the same problems by means of a different mechanism that is suitable for providing arthrodesis between either fractured parts of a bone or the cortex of a bone and the surrounding soft tissue. This invention consists of a screw-type arthrodesis device and installation tool that achieve the same functional characteristics of the prior-art devices without some of the drawbacks and with a much simpler design and relatively inexpensive costs of manufacture.